Rewrite srdwm in Rust: working X11 and Wayland backends, Lua config

The C++ prototype (moved to legacy-cpp/) was mostly a design skeleton:
X11 and Windows backends were partially real, Wayland created the
wlroots object graph but never wired a single event listener, macOS
was stub except monitor enumeration, and the Lua engine's srd.bind()
stored a key-combo string but never the actual closure. See
docs/PRIOR_ART.md for the full audit.

This replaces it with a Cargo workspace:

- srdwm-core: platform-independent window/workspace/monitor state,
  a real master-stack tiling layout, and SmartPlacement grid/cascade/
  snap-to-edge placement - fixing several bugs in the C++ version
  (hardcoded 2-column grid, cascade that never cascaded, snap-to-edge
  that always returned a fixed rect). 35 unit tests.
- srdwm-config: the srd Lua API via mlua, implementing the surface
  docs/DEFAULTS.md always documented but the C++ engine never actually
  built (srd.window.close()/focus(direction), srd.workspace.next(),
  real keybinding closures, require("srd") support). 10 unit tests.
- srdwm-x11: a real reparenting WM with a drawn title bar (buttons,
  drag, resize), verified live under Xephyr - frame placement and
  client offset match srdwm-core's computed geometry exactly, and the
  decoration renders correctly on screen.
- srdwm-wayland: a from-scratch smithay compositor (the C++ version
  had nothing working to port from) - runs via the winit backend,
  tracks xdg-shell toplevels through the same WindowManager and
  hit-testing code X11 uses, verified to start/render/run without
  crashing. Decorations are solid-color (no text yet); see
  docs/IMPLEMENTATION_STATUS.md for exact scope.
- srdwm-windows / srdwm-macos: structured, cfg-gated designs informed
  by komorebi/glazewm and yabai/AeroSpace respectively (see
  docs/PRIOR_ART.md), honestly marked as unbuilt/unverified since this
  sandbox has no Windows or macOS target.

1 files changed, 18 insertions, 0 deletions

diff --git a/crates/core/src/monitor.rs b/crates/core/src/monitor.rs
new file mode 100644
index 0000000..aa25fba
--- /dev/null
+++ b/crates/core/src/monitor.rs
@@ -0,0 +1,18 @@
+use crate::geometry::Rect;
+
+pub type MonitorId = u32;
+
+#[derive(Debug, Clone)]
+pub struct Monitor {
+    pub id: MonitorId,
+    pub name: String,
+    pub geometry: Rect,
+    pub refresh_rate_mhz: u32,
+    pub primary: bool,
+}
+
+impl Monitor {
+    pub fn new(id: MonitorId, name: impl Into<String>, geometry: Rect) -> Self {
+        Self { id, name: name.into(), geometry, refresh_rate_mhz: 60_000, primary: false }
+    }
+}